Chapter 6 Bones

Bone

An organ made up of several different tissues: bone tissue (osseous tissue), cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue

What are the 6 basic functions of bone?

1. Support 2. Protection 3. Assistance in movement 4. Mineral homeostasis 5. Blood cell production 6. Triglyceride storage

Support

Bone provides a structural framework that supports soft tissues and provides attachment points for tendons

Protection

Bone protects internal organs from injury

Assistance in movement

Skeletal muscles attach to bones. When muscles contract, they pull on the bones to produce movement

Mineral homeostasis

Bone stores and releases minerals, especially calcium. About 99% of the body's calcium is stored in bone. Bone can release minerals into the blood when they are needed

Blood cell production

Red bone marrow produces red blood cells, white blood cells, and platelets through hemopoiesis (blood making)

What does red bone marrow contain?

Developing blood cells, adipocytes, fibroblasts, and macrophages. It also contributes to bone development in fetuses

Hemopoiesis

The process of producing blood cells ("blood making") in red bone marrow

Triglyceride storage

Yellow bone marrow, which is made primarily of adipose cells, stores triglycerides that can serve as a potential chemical energy reserve

Diaphysis

The shaft or body of a long bone. It is long and cylindrical and is the portion between the proximal and distal ends

Epiphysis

The growing ends of a long bone. There is a proximal epiphysis and a distal epiphysis

Metaphysis

The region between the diaphysis and epiphysis. Each metaphysis contains an epiphyseal plate

Epiphyseal plate

A layer of hyaline cartilage in the metaphysis that allows the diaphysis to grow in length

What happens to the epiphyseal plate when bone growth is complete?

The cartilage is replaced with bone, forming the epiphyseal line

Epiphyseal line

The remnant of the epiphyseal plate after the bone has finished growing in length

Articular cartilage

Articular cartilage lacks a perichondrium and blood vessels, limiting its ability to repair itself

Periosteum

A tough connective tissue sheath surrounding the outer surface of bone that is not covered by articular cartilage. It contains an associated blood supply

What are the two layers of the periosteum?

An outer fibrous layer and an inner osteogenic layer

Outer fibrous layer

The outer layer of the periosteum, made of dense irregular connective tissue

Inner osteogenic layer

The inner layer of the periosteum containing cells that enable bone to grow in thickness

Functions of the periosteum

Protects bone, assists with fracture repair, provides nourishment, and provides attachment points for ligaments and tendons

Medullary cavity

The hollow, cylindrical space within the diaphysis of a long bone

What is found in the medullary cavity?

Yellow bone marrow and blood vessels in adults

Why does the medullary cavity help reduce bone weight

The hollow cavity reduces the amount of dense material needed, making the bone lighter

Endosteum

A membrane within the bone that lines the medullary cavity. It contains a single layer of bone-forming cells and connective tissue

What is the extracellular matrix of bone tissue like?

Bone tissue contains an abundant extracellular matrix that surrounds widely separated cells

What is the most abundant mineral salt in bone?

Calcium phosphate

What happens to calcium phosphate in bone?

Calcium phosphate combines with calcium hydroxide to form hydroxyapatite crystals

What are hydroxyapatite crystals?

Mineral crystals formed from calcium phosphate and calcium hydroxide that contribute to the hardness of bone

What combines with the mineral crystals in bone?

The crystals combine with other mineral salts and collagen fibers in the extracellular matrix

Why are collagen fibers important in bone?

Collagen fibers are incorporated into the extracellular matrix along with mineral crystals, contributing to the structure and properties of bone

What are the 4 types of cells in bone tissue?

Osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts

Osteoprogenitor cells

Unspecialized bone stem cells derived from mesenchyme. They are the only bone cells that undergo cell division. The resulting cells develop into osteoblasts

Where are osteoprogenitor cells found?

Along the inner portion of the periosteum, in the endosteum, and in canals within bone that contain blood vessels

Osteoblasts

Bone-building cells that synthesize and secrete collagen fibres and initiate calcification

What happens to osteoblasts when they become surrounded by bone matrix?

They become trapped within the matrix and develop into osteocytes

Osteocytes

Mature bone cells and the main cells found in bone tissue. They maintain the metabolism of bone tissue

Osteoclasts

Large cells derived from the fusion of monocytes. They are concentrated in the endosteum and break down bone tissue

What do osteoclasts do?

They release lysosomal enzymes and acids that digest the protein and mineral components of the bone matrix. This process is called bone resorption

Bone resorption

The process by which osteoclasts break down bone matrix, releasing its mineral and protein components

Compact bone tissue

The strongest form of bone tissue. It contains relatively few spaces and provides protection, support, and resistance to stresses from weight and movement.

Where is compact bone found?

Beneath the periosteum of all bones and makes up most of the diaphysis (shaft) of long bones

What is compact bone composed of?

Osteons (Haversian systems), concentric lamellae, lacunae, canaliculi, interstitial lamellae, perforating (Volkmann) canals, and circumferential lamellae

Spongy bone tissue

Also called trabecular or cancellous bone. It is lighter than compact bone, provides support and protection for red bone marrow, and does not contain osteons

Where is spongy bone located?

In the interior of a bone, where it is protected by a layer of compact bone

What is spongy bone made of?

It consists of thin columns or plates of bone called trabeculae

Trabeculae

Thin irregular columns or plates of bone

What is found in the spaces between trabeculae?

Red or yellow bone marrow

Does spongy bone contain osteons?

No

How well supplied with blood are bones?

Bone is rich in blood vessels, especially in areas containing red bone marrow

Periosteal arteries

Small arteries accompanied by nerves that enter the diaphysis through Volkmann's canals and supply the periosteum and outer compact bone

Where do periosteal arteries enter the bone?

They enter the diaphysis through Volkmann's canals

Nutrient artery

A large artery that enters the diaphysis through a hole in compact bone called a nutrient foramen

What happens to the nutrient artery after entering the medullary cavity?

It divides into proximal and distal branches

What does the nutrient artery supply?

It supplies the bone marrow and internal bone tissue

Nutrient foramen

A hole/opening in compact bone through which the nutrient artery enters the bone

Metaphyseal arteries

Arteries that enter the metaphyses of long bones. Along with the nutrient artery, they supply red bone marrow and bone tissue of the epiphyses

What accompanies the blood vessels that supply bone?

Nerves accompany the blood vessels that supply the bone, including the periosteum

Bone formation (ossification)

The process of forming bone. It occurs during initial formation of the skeleton in the embryo and fetus, growth of bones during infancy, childhood, and adolescence, bone remodelling, and bone repair

What are the 2 patterns of bone formation?

Intramembranous ossification and endochondral ossification

Intramembranous ossification

Bone formation that occurs directly within mesenchyme. It produces relatively simple bones, including many flat bones of the skull, facial bones, and clavicles

Endochondral ossification

Bone formation that involves the replacement of a cartilage model with bone. It is responsible for forming most bones of the body

How does a long bone grow in length?

It grows through cell division of chondrocytes and continued growth of cartilage at the epiphyseal plate

What is the epiphyseal plate?

A plate of hyaline cartilage located between the diaphysis and epiphysis that allows a long bone to grow in length

How does a bone grow in thickness?

Through appositional growth, in which new bone is added to the outer surface of the bone

What happens during appositional growth?

Osteoblasts in the periosteum deposit new bone on the outer surface while bone is removed from the inner surface, helping enlarge the medullary cavity

Bone remodelling

The ongoing replacement of old bone tissue with new bone tissue

Bone resorption

The removal of minerals and collagen fibers from bone by osteoclasts

Bone deposition

The addition of minerals and collagen fibres to bone by osteoblasts

What minerals are especially important for bone growth?

Large amounts of calcium and phosphorus are needed while bones are growing. Smaller quantities of magnesium, fluoride, and manganese are also needed

What is the role of Vitamin A in bone growth?

Vitamin A is important for osteoblast activity

What is the role of Vitamin C in bone growth?

Vitamin C is important for collagen synthesis

What is the role of Vitamin D in bone growth?

Vitamin D helps build bone through calcium absorption

What are Vitamin K and Vitamin B12 important for?

They are important for the synthesis of bone proteins

How do insulin-like growth factors affect bone growth?

During childhood, insulin-like growth factors stimulate osteoblasts, promote cell division, and enhance protein synthesis

How do thyroid hormones and insulin affect bone growth?

They also help stimulate bone growth and development, including effects on osteoblast activity, cell division, and protein synthesis

How do sex hormones affect bone growth?

At puberty, estrogens and androgens increase osteoblast activity and bone extracellular matrix production, contributing to the teenage growth spurt

What are the major sex hormones involved in bone growth?

Estrogens and androgens, such as testosterone. Both sexes have both types, although the levels differ